1. Field of the Invention
The present invention relates generally to an optical waveguide structure for an optical communication system, and particularly to a planar photonic crystal waveguide for implementing a variety of optical functions in an optical communication system.
2. Technical Background
Photonic crystals are periodic optical materials. The characteristic defining a photonic crystal structure is the periodic arrangement of dielectric or metallic elements along one or more axes. Thus, photonic crystals can be one-, two-, and three-dimensional. Most commonly, photonic crystals are formed from a periodic lattice of dielectric material. When the dielectric constants of the materials forming the lattice are different (and the materials absorb minimal light), the effects of scattering and Bragg diffraction at the lattice interfaces control the propagation of optical signals through the structure. These photonic crystals can be designed to prohibit optical signals of certain frequencies from propagating in certain directions within the crystal structure. The range of frequencies for which propagation is prohibited is known as the photonic band gap.
An exemplary two dimensional photonic crystal which is periodic in two directions and homogeneous in a third is shown in FIG. 1. More specifically, the photonic crystal 10 is fabricated from a volume of bulk material 12 having a square lattice of cylindrical air-filled columns 14 extending through the bulk material in the z-axis direction and periodic in the x-axis and y-axis directions. For normal theoretical analysis and modeling, the photonic crystal 10 has conventionally been assumed to be homogeneous and infinite in the z-axis direction. In this exemplary figure, the plane of the two dimensional photonic crystal is the xy plane.
Another exemplary photonic crystal is shown in FIG. 2. The photonic crystal 15 is similar to the photonic crystal 10, but the cylindrical air-filled columns are disposed in a hexagonal array. A third exemplary two-dimensional photonic crystal is shown in FIG. 3. The photonic crystal 16 is also similar to photonic crystal 10, but consists of an array of dielectric columns 18 in an air background.
The propagation of optical signals in these structures is determined by a variety of parameters, including, for example, radius of the columns, pitch (center-to-center spacing of the columns) of the photonic crystal, structural symmetry of the crystal (e.g. square, triangular, hexagonal, rectangular), and refractive indexes, (such as the index of the material of the columns and the index of the bulk material exterior to the columns). FIG. 4 shows the photonic band diagram for a hexagonal array of air-filled columns 14 in a dielectric bulk material 12. One skilled in the art will appreciate that there is a range of photon frequencies, known as the photonic band gap, for which propagation in the plane of the photonic crystal is prohibited. This photonic band gap, denoted by region 19, is determined by the structure of the photonic crystal, especially by the parameters listed above.
A defect can be introduced into the crystalline structure for altering the propagation characteristics and localizing the allowed modes for an optical signal. For example, FIG. 5 shows a two-dimensional photonic crystal 20 made from a dielectric bulk material with a square lattice of air-filled columns 22 and a linear defect 24 consisting of a row of missing air-filled columns. The band diagram for this photonic crystal structure is shown in FIG. 6. The photonic band gap is denoted by the region 30, while a band of allowed guided modes associated with the defect is denoted by the very thin region 32. The exact position and shape of the region 32 on the graph of FIG. 6 depends upon the photonic crystal parameters. Physically, this means that while optical signals of a given frequency are prohibited from propagating in the bulk photonic crystal 20, they may propagate in the defect region 24. An optical signal, whether a pulse or a continuous wave, traveling in the defect region 24 may not escape into the bulk photonic crystal 20, and so is effectively waveguided in the defect region 24. For a given wavevector, the region 32 only encompasses a narrow band of frequencies. Optical signals of a given wavevector must have frequencies within this narrow band in order to be guided in the defect 24. In the theoretical case of the infinitely thick two-dimensional photonic crystal, light is not confined in the z-axis direction by the photonic crystal structure. While the defect in the above example is a constructed from a row of missing air-filled columns, other defect structures are possible. For example, a defect may consist of one or more columns of a different shape or size than those of the bulk photonic crystal.
Additionally, the crystal structure can be composed of several photonic crystal regions having different parameters, in which case the defect is located at the border between the two regions. Such a structure is shown in FIG. 7, in which the photonic crystal structure 40 has a first photonic crystal region 42 and a second photonic crystal region 44. In the example of FIG. 8, in the first region 42 the cylindrical columns have radius R1 and are arranged with a pitch P1. In the second region, the photonic crystal structure has different parameters, with a column radius of R2 and a pitch of P2. This photonic crystal also has a photonic bandgap, with the possibility of a defect mode for allowing propagation of an optical signal. Because of this defect mode phenomenon and its dependence on the photonic crystal parameters, it is possible to control the propagation of an optical signal in a defect waveguide by controlling the parameters associated with the photonic crystal regions.
Since an optical signal propagating in a defect waveguide is prohibited from propagating in the bulk photonic crystal, it must follow the waveguide, regardless of the shape of the defect waveguide. An advantage of such a structure is that waveguides with a very small bend radius on the order of several wavelengths or even less are expected to have a very low bend loss, since an optical signal is prohibited from escaping the defect waveguide and propagating in the surrounding photonic crystal. FIG. 8 shows the results of a simulation of propagation in a 2D photonic crystal wherein substantially all of the optical signal successfully navigates a 90xc2x0 bend with a radius of curvature smaller than the wavelength of the optical signal. Likewise, waveguide splitters and combiners are expected to have low radiation losses. FIG. 9 shows a 180xc2x0 splitter in which nearly 100% transmission is achieved with the optical signal from the input guide 60 perfectly split into the two branches 62 and 64. In this case, a pair of small columns was added in order to reduce the small fraction of light that was backreflected into the input guide 60.
In-plane confinement by a photonic crystal defect waveguide can be confined with refractive confinement in the dimension normal to the photonic crystal to provide a defect channel waveguide. This is most commonly achieved by providing a thin slab of a two-dimensional photonic crystal (known as a planar photonic crystal) having a defect waveguide with lower refractive index materials above and below the photonic crystal waveguide. For example, FIG. 10 shows the structure of a planar photonic crystal defect waveguide 70 with a core layer 71, an underclad layer 72, and an overclad layer 74, all of which include a photonic crystal structure defining the defect channel waveguide. For use herein, an effective refractive index of a material is defined as the volume average refractive index of that material. In order to provide vertical confinement, the effective refractive index of the core layer 71 is higher than the effective refractive indices of the underclad layer 72 and the overclad layer 74. This structure may be made by etching an array of columnar holes into a slab waveguide containing a core layer, an underclad layer, and an overclad layer.
An example of an alternative structure appears in FIG. 11. In this case, only the higher effective refractive index core layer 81 has the photonic crystal structure; the underclad 82 and the overclad 84 are homogeneous. In this structure, which may be fabricated by bonding a thin slab of material containing the 2D photonic crystal structure to a substrate, the substrate serves as the underclad, and the overclad is air Alternative structures have been envisioned wherein a free-standing planar photonic crystal is clad on both sides by air, or wherein both the underclad and overclad are a dielectric material.
FIG. 12 shows another alternative structure, having both the core layer 90 and the underclad layer 92 patterned with a two dimensional photonic crystal structure, and a homogeneous overclad 91. This structure can be made by etching an array of columnar holes into a slab waveguide having an optically homogeneous core layer deposited onto a optically homogeneous underclad layer. In both of these alternative architectures, the upper cladding may be air, or it may be a layer of dielectric material.
In all three architectures, an optical signal is constrained in the defect waveguide vertically by total internal reflection, and horizontally by the photonic band gap. Passive waveguiding has been predicted by optical simulations and demonstrated in experimental systems in all three architectures. Calculations for a planar photonic crystal waveguide have been described in Kuchinsky et al., xe2x80x9c3D localization in a channel waveguide in a photonic crystal with 2D periodicity,xe2x80x9d Optics Communications 175, p. 147-152 (2000), which is hereby incorporated by reference. The calculation method uses a numerical solution of the full vector Maxwell equations, in which the electromagnetic modes are expanded in a sum of plane waves. This approach is well suited to periodic photonic crystals. When the physical system lacks periodicity, for example as in the z-direction of a bulk photonic crystal or the transverse direction of a defect waveguide, then a supercell is employed in which a periodic array of crystals or waveguides is considered. The artificial repeat distance of this supercell is kept large enough to avoid unwanted calculation artifacts. The supercell method is a standard approach that allows periodic band structure computer codes to salve nonperiodic systems. Solution of the fall vector Maxwell equations required, as the simpler scalar approximation gives incorrect results due to the large dielectric/air index. Propagation through sharp defect waveguide bends has also been predicted and experimentally demonstrated.
Active devices may be based on planar photonic crystal defect channel waveguides. For example, an actively controllable Y junction is shown in FIG. 13. The Y junction has an input waveguide 94, a first output waveguide 95, and a second output waveguide 96. The output waveguides are modified by the presence of controllable lattice sites 98 located in the regions 97 of the output waveguides near the branch point and comprising cylindrical columns formed of a ferrite material to which a variably controllable external electromagnetic field may be applied. The locations of the controllable lattice sites conform to the column and row positions of the surrounding lattice region and in effect form an extension of the lattice. Control of the controllable lattice sites 98 is effected such as to vary the refractive index of the ferrite material, and therefore the propagation characteristics of the defect waveguides. The presence of the controllable lattice sites can in effect be turned on or off in variable number to thereby variably control the effective apertures of the output waveguides 95 and 96. This is represented in FIG. 13 by showing only those controlled lattice sites 98 which are turned xe2x80x9conxe2x80x9d and which in this example are shown only in the second output waveguide 96. The amount of optical signal coupled into the second output waveguide 96 is thereby controllable by setting the number of sites which are turned on, the remainder of the optical signal being diverted into the second waveguide 96. Active photonic crystal materials and devices with bandgaps in the near infrared, however, would be difficult to fabricate using ferrite materials.
It is also possible to externally control the propagation of an optical signal in a planar photonic crystal defect channel waveguide by varying the refractive index of the bulk material of the planar photonic crystal. The externally applied control may be one of a number of available options including the application of local heating, the injection of electrical current into a semiconductor bulk material, or other suitable optically, electromagnetically or electromechanically induced effects. The photonic crystal lattice is substantially unaffected by this control and continues to serve as a means of confining the optical signal within the waveguide so as to pass through the controlled dielectric region. These types of devices are unattractive in that the photonic crystal must be formed in a thermo-optically, electro-optically, or mechano-optically active material, limiting the choice of device materials and fabrication processes.
Accordingly, photonic crystal waveguide devices which can perform a wide variety of optical transformations and are amenable to a wide variety of materials and manufacturing processes are desired.
One aspect of the present invention relates to a planar photonic crystal defect waveguide device in which an optical signal is confined in the plane of the photonic crystal by the photonic bandgap, and in the direction normal to the photonic crystal by the upper clad region and the lower clad region, wherein the propagation of light in the waveguide is controlled by varying the optical properties of either the upper clad region or the lower clad region or both.
Another aspect of the present invention relates to a method for controlling an optical signal propagating in such a photonic crystal defect channel by changing the optical properties of either the upper clad region and the lower clad region or both.
Another aspect of the present invention relates to an optical device for controlling an optical signal including a planar photonic crystal structure having a defect waveguide, an upper clad region continuous with the top surface of the defect waveguide, and a lower clad region continuous with the bottom surface of the defect waveguide, wherein at least one of the upper clad region or the lower clad region is a controllable region having a controllable optical property sufficient to modify the optical signal.
Another aspect of the present invention relates to a method for controlling an optical signal including providing a device including a planar photonic crystal structure having a defect waveguide, an upper clad region continuous with the top surface of the defect waveguide, and a lower clad region continuous with the bottom surface of the defect waveguide, wherein at least one of the upper clad region or the lower clad region is a controllable region having a controllable optical property sufficient to modify the optical signal; launching the optical signal into the defect waveguide; and controlling the optical property of at least one of the upper clad region or the lower clad region so as to effect a change in the propagation of the optical signal in the defect waveguide.
The device of the present invention results in a number of advantages. Active planar photonic crystal defect waveguide devices may be designed and fabricated with well-defined guiding characteristics in all three dimensions, and may have modes with zero group velocity. The active planar photonic crystal defect waveguides may be fabricated by standard semiconductor manufacturing techniques. The devices of the present invention do not derive their activity from an active photonic crystal waveguide core, and so may be made from photonic crystals of any standard passive waveguide material. The refractive indices of the upper clad region or the lower clad region or both may be varied in many ways, including by a thermo-optic effect, an electro-optic effect, a mechano-optic effect, or by physically introducing a different material into the clad region. The devices of the present invention may affect various optical transformations, including attenuation, modulation, and switching, all with the reduced device size afforded by the efficiency of tight photonic crystal waveguide bends.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) of the invention, and together with the description serve to explain the principles and operation of the invention.